Understanding Instrument Output Signals: A Comprehensive Guide for Industrial Applications

In modern industrial production and scientific research, instruments play a crucial role in measuring and indicating various physical quantities. The type of output signal directly impacts data acquisition, processing, and subsequent control decisions. With the continuous development of technology, the types of instrument output signals have become increasingly diverse, ranging from traditional analog signals to digital signals and various bus signals. Each signal type has its unique application scenarios, advantages, and disadvantages. In this blog, we will explore the main types of output signals.

Digital Signals

Digital signal output typically employs communication protocols such as HART, WirelessHART, FOUNDATION Fieldbus, PROFIBUS, and Modbus for data transmission. These protocols offer high transmission rates and reliable data, making them suitable for applications where multiple transmitters need to be connected into a network for multi-point collection and centralized management. They also support remote monitoring and diagnostic functions, facilitating easier maintenance and management.

Advantages

• High Precision: Digital signals represent measurement results using discrete values, providing higher anti-interference ability and precision compared to analog signals. During transmission, digital signals are less susceptible to noise and interference, ensuring the accuracy and reliability of measurement results.
• Multiple Communication Protocol Options: These protocols enable instruments to communicate with different types of control systems and devices, meeting various industrial automation requirements.
• Support for Multi-Variable Measurement: Some advanced instruments can measure multiple variables simultaneously and output the results via digital signals. For example, certain flow meters can measure mass flow, volumetric flow, standard volumetric flow of gas, density, temperature, and other parameters. This multi-variable measurement capability provides more comprehensive process information, aiding in optimizing production processes and improving product quality.
• Intelligent Diagnostic Tools: Many digital instruments are equipped with advanced diagnostic tools that can monitor the instrument's working status and performance in real-time. These tools can identify and predict potential failures, providing corresponding alarm and diagnostic information. This allows maintenance personnel to detect and solve problems promptly, preventing production interruptions and safety accidents.

Disadvantages

• System Instability Due to Instrument Failure: In case of a failure, such as a malfunction in the digital signal processing unit, it may lead to abnormal or distorted output signals, thereby affecting the stability and accuracy of the entire system.
• High Requirements for Hardware and Software: Digital instruments usually require high-quality hardware and software to support their operation. In terms of hardware, high-precision sensors and converters are needed to ensure signal accuracy and stability. Software-wise, powerful data processing and communication capabilities are required to support digital signal transmission and processing. These requirements increase the complexity and cost of the system.
• Challenges in System Maintenance and Upgrades: As technology advances, the functions and performance of digital instruments are constantly improving. However, this also brings challenges to system maintenance and upgrades. To ensure the normal operation and performance optimization of the system, regular maintenance and upgrades of digital instruments are necessary, which may require significant investments of manpower, materials, and financial resources and may affect the system's uptime.

Analog Signals

Instrument analog signals are continuously changing physical quantities used to represent the values of measured objects. In industrial automation systems, analog signals are usually converted into standard current or voltage signals by sensors or transmitters for transmission and processing.

Common Types

• Current Signals: Common analog current signals include 4 - 20mA and 0 - 20mA. Among them, the 4 - 20mA signal is widely used due to its strong anti-interference ability and long transmission distance.
• Voltage Signals: Common analog voltage signals include 0 - 5V, 0 - 10V, and 1 - 5V. Voltage signals are prone to interference during transmission and are usually used for short-distance transmission or in low-noise environments.

Advantages

• Stable Transmission: Under appropriate transmission media and conditions, analog signals can be transmitted relatively stably, maintaining the accuracy and integrity of the signals.
• Strong Compatibility: Analog signals are compatible with a variety of devices and systems and can be easily connected to various sensors, transmitters, and controllers.
• Continuous Change and High Precision: Analog signals can continuously represent the values of measured objects, offering high precision and resolution.
• Relatively Low Cost: Compared to digital signals, the transmission and processing equipment for analog signals is relatively simple, resulting in lower costs. This makes analog signals advantageous in some applications with high cost requirements.

Disadvantages

• Susceptible to Interference: Analog signals are easily affected by external interference such as electromagnetic interference and noise during transmission. These interferences can cause signal distortion or misreading, affecting the accuracy and reliability of the system. To address this issue, additional anti-interference measures such as using shielded cables and signal isolators are usually required.
• Limited Transmission Distance: The transmission distance of analog signals is limited by factors such as signal attenuation and noise. When the transmission distance is too long, the strength and precision of the signal may be affected, leading to a decline in system performance.
• Precision Affected by Multiple Factors: The precision of analog signals is influenced by various factors such as transmission medium loss, environmental temperature changes, and equipment aging. These factors can cause a decrease in signal precision, thereby affecting the measurement and control precision of the system. To improve the precision of analog signals, a series of measures such as using high-quality transmission media and regularly maintaining and calibrating the equipment are needed.

Switch Signals

Switch signals represent two different states, usually denoted by "0" and "1" or "on" and "off". In industrial automation systems, switch signals are commonly used to indicate the operating status of equipment, alarm signals, and control instructions.

Advantages

• Fast Response Speed: Switch signals, which typically represent the operating status or control instructions of equipment, have clear "on" and "off" states. This enables them to quickly trigger corresponding control logic during transmission and processing, achieving a fast response.
• High Reliability: The transmission of switch signals is usually realized through relay contacts or level signals, which have high stability and reliability. Compared to analog signals, switch signals are less affected by external interference and can maintain the accuracy and integrity of the signals.
• Easy to Integrate and Expand: Switch signals have a simple logical relationship and clear state representation, making them easy to integrate and expand with other devices and systems. In industrial automation systems, switch signals can be conveniently connected to other sensors, controllers, and actuators to implement complex control logic and automation functions.
• Low Cost: Compared to analog signals, the transmission and processing equipment for switch signals is relatively simple and inexpensive. This makes switch signals advantageous in some industrial automation applications with high cost requirements. Additionally, due to their high stability and reliability, the maintenance cost is also relatively low.

Active and Passive Signals

The distinction between active and passive signals depends on whether the device providing the 4 - 20mA signal has an independent power supply line. If it does, the output signal is an active signal; otherwise, it is a passive signal.

At VACORDA, we have 29 years of experience in the industrial measurement field and hold 47 patents. Our product line includes liquid level meters, flow meters, switches, pressure sensors, pressure transmitters, and safety barrier product series, all of which are certified to meet international safety standards. We are committed to providing high-quality products and solutions to meet the diverse needs of our customers in the industrial sector.